Plasma processing is often used under circumstances where it is beneficial to restrict the presence of the plasma to a certain plasma-containing region in a processing chamber while maintaining the ability to flow gases through the plasma-containing region. Several methods of plasma containment are known, including, for example, magnetic confinement, inertial confinement, and structural confinement by solid or nearly-solid barriers. Inertial confinement is operable for only short periods of time and has been shown useful only in power generating and explosive devices. Magnetic confinement has strong limitations in its applicability to many industrial applications. Because of these and other shortcomings of inertial and magnetic confinement systems, structural confinement using physical barriers is often used in industrial plasma confinement.
Known, conventional devices are arranged such that a substrate is positioned within a processing chamber and the plasma is formed between the substrate and an opposing side of the processing chamber, e.g., a processing surface of the substrate faces the plasma. This is most often achieved with a processing gas being injected from the opposing side and exhausted from the substrate side of the processing chamber by a turbo pump. However, at vacuum pressures the exhaust flow is slow, and if plasma is drawn from the plasma-containing region into the pump, then the plasma will damage the pump and/or cause other problems.
FIG. 1 illustrates one such plasma processing device 10. The particular arrangement of plasma processing device 10 with a gas injection assembly 12 and an exhaust assembly 14 as shown in FIG. 1 is prone to various problems. The device 10 comprises a processing chamber 16 configured to facilitate the formation of plasma 18 in a process space 20, a substrate holder 22 positioned within the plasma processing device 10 and configured to support a substrate 24 thereon. A vacuum pump 26 is coupled to the processing chamber 16 via pumping port 28 and configured to evacuate process gases from the process space 20.
Additionally, the plasma processing device 10 includes a plasma generation system 30 coupled to the plasma processing chamber 16 and configured to form the processing plasma 18 from a process gas injected into the process space 20.
A controller 38 is operably coupled to one or more of the plasma generating system 30, the vacuum pump 26, the gas injection assembly 12, and other components of the device 10 for controlling plasma processing of the substrate 24.
To overcome the problem associated with drawing plasma from the process space 20 and into the pump 26, the exhaust assembly 14 of FIG. 1 further includes a first exhaust baffle 32 positioned over the exhaust port 28. The first baffle 32 includes a plurality of openings extending therethrough, each of which has a diameter that is larger than one Debye length of the plasma 18. One of ordinary skill in the art would readily appreciate that the Debye length is the length over which the plasma 18 screens out electric fields. As such, the first baffle 32 allows the plasma 18 to pass through the openings. This quiescent plasma 34 between the first baffle 32 and the pumping port 28 is less intense than the main plasma 18 as it is not ignited by RF excitation but due to diffusion from the main plasma 18.
The exhaust assembly 14 further includes a second baffle 36 positioned between the first baffle 32 and the pumping port 28 and having a plurality of holes that are less than the Debye length. The quiescent plasma 34, being less intense than the main plasma 18, is quenched as it passes through the sub-Debye openings of the second baffle 36, rendering the exhaust assembly 14 stable and protecting the pump 26.
However, the device 10 as shown in FIG. 1 does not resolve the issues associated with slow exhaust. Therefore, there remains a need for a plasma processing system that protects the vacuum pump 26 from plasma damage while overcoming the slow exhaust.